Should Sighs be Used in ARDS?
August 1, 1999 5 minutes read
Should Sighs be Used in ARDS?
Abstract & CommentaryA "lung-protective strategy" is currently in vogue for managing mechanical ventilation in patients with acute respiratory distress syndrome (ARDS), and evidence is accumulating that it may improve outcomes in this condition. Management, according to this approach, includes using enough positive end-expiratory pressure (PEEP) to keep the lung open (i.e., to maintain tidal ventilation above the lower inflection point on the pressure-volume curve of the relaxed respiratory system), reducing tidal volume (VT) to 5 or 6 mL/kg in order to reduce the likelihood of lung overdistension, and limiting end-inspiratory plateau pressure to less than 35 cm H2O in order to decrease ventilator-induced lung injury.
This study by Pelosi and associates at the University of Milan tested the hypotheses that using the above ventilatory strategy does not reverse all the atelectasis present in the lungs of patients with ARDS, and that application of intermittent large-volume sigh breaths would further recruit the lung, as shown by improved arterial oxygenation. Pelosi et al studied 10 patients with ARDS, five with pneumonia as the primary diagnosis (ARDS of pulmonary origin), and five with abdominal sepsis, pancreatitis, or osteomyelitis (ARDS of nonpulmonary origin). The patients, who were paralyzed and heavily sedated, first underwent two hours of ventilation using the "lung-protective strategy" (mean VT, 560 mL) without sighs. During the following hour they were given three consecutive sighs/min at 45 cm H2O plateau pressure (mean VT, 1100 mL), with other VT reduced such that minute ventilation remained the same. Following this "sigh" hour, the patients were ventilated with an additional hour at the baseline settings. Total PEEP (mean 14 cm H2O) and other ventilator settings were unchanged throughout the study period.
After one hour of ventilation with sigh breaths, mean arterial PO2 increased from 93 to 138 mmHg. Venous admixture decreased from a mean of 38% to 28%, with a slight fall in arterial PCO2. End-expiratory lung volume increased about 400 mL during the "sigh" hour, and this correlated with changes in oxygenation and lung elastance. All of the findings were statistically significant. One hour after the sigh breaths were discontinued, all the findings had returned to their baseline values. The findings were more pronounced in patients with ARDS of extrapulmonary than of pulmonary origin. Pelosi et al conclude that the currently popular "lung-protective strategy" for ventilatory management of ARDS may not optimize lung recruitment or arterial oxygenation. They further conclude that the application of sigh breaths may improve lung recruitment and arterial oxygenation. (Pelosi P, et al. Am J Respir Crit Care Med 1999;159:872-880.)
Comment by David J. Pierson, MD, FACP, FCCP
This is an interesting study, especially for anyone who has been involved in mechanical ventilation for a long time, and its conclusions seem a bit hard to reconcile with the philosophy of lung protection in managing ARDS. In the 1960s it was observed that mechanical ventilation with the "physiologic" tidal volumes observed in healthy, spontaneously breathing persons (5-7 mL/kg) led to progressive microatelectasis and increased P(A-a)O2 if intermittent sigh breaths were not provided. The use of larger tidal volumes—12-15 mL/kg—without sighs was also effective in preventing these adverse effects. As a result of these studies in healthy persons undergoing anesthesia for elective surgery, the "default" VT for mechanical ventilation in the ICU became 12-15 mL/kg. Sigh breaths rapidly went out of fashion, and a 1992 review of their use concluded that "based on the available literature, the sigh breath really has no place in the intensive care unit" (Branson RD, et al. Respir Care 1992;37:462-468). In part this was because of an increasing awareness of ventilator-induced lung injury and the association of clinical barotrauma with high peak airway pressures in patients with ARDS.
Animal experiments during the 1980s demonstrated ventilator-induced lung injury when high distending pressures and volumes were used in normal animals, and it was subsequently shown that this injury was accentuated when acute lung injury was already present. As a result of these and other studies, the "lung-protective strategy" was developed, which includes the use of smaller VT in the original "physiologic" range. A recent multicenter comparison of "lung-protective" ventilation vs. management that included ventilation with more traditional tidal volumes was stopped early because of significantly lower mortality in patients managed with the new approach, according to summary results presented at the recent American Thoracic Society meeting in San Diego. These findings support the earlier results of Amato and colleagues (N Engl J Med 1998;338:347-354) that a ventilatory strategy that includes avoiding lung overdistension improves survival in ARDS.
With this study it would appear that we have come full circle. We originally used sigh breaths to avoid or reverse atelectasis in patients ventilated with small VT. We then observed that the use of large distending volumes produced lung injury in animals and found that ARDS patients managed with small VT and no sighs (along with other differences from traditional ventilator settings and new blood gas targets) had better survival than those managed with larger VT. A management strategy subsequently evolved that included the avoidance of plateau pressures greater than 35 cm H2O and kept VT less than 7 mL/kg. Now, Pelosi et al demonstrate that repeated inflation at 45 cm H2O and twice the usual VT improves lung recruitment and arterial oxygenation.
Pelosi et al acknowledge two important aspects of this study that should prevent clinicians from immediately introducing sigh breaths into the ventilatory management of ARDS. The study population was small. More important, the observation period was only four hours total, during which time sighs were applied for only one hour. Improved PaO2 and lung compliance do not necessarily mean better outcomes. The clinical effects over many hours to days of administering three sigh breaths per minute at twice the currently recommended VT are unknown. For sighs to re-enter clinical practice, particularly in the management of ARDS, would require the results of a controlled clinical trial demonstrating that ventilation with sigh breaths was not associated with the same ventilator-induced lung injury and increased mortality that the "lung-protective strategy" was designed to prevent.
The study by Pelosi et al shows that adding sigh breaths to the "lung protective strategy" for managing ARDS:
a. improves survival.
b. reduces the incidence of ventilator-induced lung injury.
c. improves arterial PO2.
d. All of the above
e. None of the above
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